Programmable delay clock gaters

ABSTRACT

A non-overlapping clock generator that has its dead time adjustable without a complete re-design and re-fabrication. Certain terminals of certain devices of the non-overlapping clock generator are connected only by metal layers. This allows the circuit of the non-overlapping clock generator to be changed, adjusting the dead time, by changing only the masks used to fabricate the metal layers. This allows non-overlapping clock generators on wafers that have been partially fabricated to have their dead times altered from the original design.

FIELD OF THE INVENTION

[0001] The present invention relates generally to clock buffer circuits used in VLSI integrated circuits, and, more particularly to a qualified non-overlapping clock generator/buffer.

BACKGROUND OF THE INVENTION

[0002] Qualified non-overlapping generators (also known as clock gaters) may be used to prevent data from “racing” from one latch to another thereby causing errors. Non-overlapping clock signals generated by clock gaters are typically clock signals which one clock signal has a rising edge that occurs after a falling edge of the other clock signal and a falling edge that occurs before a rising edge of the other clock signal. Such clock signals prevent races through latches by deactivating a subsequent stage before data is allowed to propagate through the current stage.

[0003] Several versions of clock gaters, and their use in an overall clocking scheme, have been presented in U.S. Pat. No. 5,124,572 to Mason et al., U.S. Pat. No. 5,306,962 to Lamb, U.S. Pat. No. 5,726,596 to Perez, U.S. Pat. No. 5,760,610 to Naffziger, and U.S. Pat. No. 5,701,335 to Neudeck. These documents are all hereby incorporated herein by reference.

[0004] One of the design tradeoffs encountered when using clock gaters involves the amount of time that both clocks are inactive (i.e. non-overlapping). This is also referred to as dead time. Decreasing the amount of dead time may increase the frequency of operation of the integrated circuit (IC) by allowing more time for circuits to evaluate when at least one clock is active. However, this decreased dead time also increases the risk that the circuit will have some timing errors due to unforeseen delays or race conditions in the design of the IC. With larger dead times, it is less likely that an unforeseen delay or race condition will cause errors.

[0005] Increasing the amount of dead time decreases the likelihood that unforeseen delays or race conditions will cause errors. However, the increased dead time reduces the amount of time available during a clock phase for circuitry to evaluate. This decreases the performance of the integrated circuit by requiring lower clock frequencies to provide the same amount of evaluate time per clock phase. These tradeoffs are typically made during the design of the integrated circuit and therefore typically require a re-design of the clock gaters and re-fabrication of the entire chip to increase or decrease dead time thereby getting the chip to function properly or improve performance, respectively.

[0006] Accordingly, there is a need in the art for a reduced cost way of manipulating the amount of dead time provided by clock gaters. The amount of dead time should be able to be manipulated on a circuit-by-circuit basis without having to re-design every clock gater and re-fabricate the entire chip from scratch.

SUMMARY OF THE INVENTION

[0007] The invention provides a non-overlapping clock generator that has its dead time adjustable without a complete re-design and re-fabrication. Certain terminals of certain devices of the non-overlapping clock generator are connected only by metal layers. This allows the circuit of the non-overlapping clock generator to be changed, adjusting the dead time, by changing only the masks used to fabricate the metal layers. This allows non-overlapping clock generators on wafers that have been partially fabricated to have their dead times altered from the original design.

[0008] Other aspects and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009]FIG. 1 is a schematic illustration of a non-overlapping clock generator having a metal mask adjustable dead time.

[0010]FIGS. 2A, 2B, and 2C are a schematic illustrations of three P/N ratios obtainable on the input inverter of the non-overlapping clock generator.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011]FIG. 1 is a schematic illustration of a non-overlapping clock generator having a metal mask adjustable dead time. Box 100 surrounds an input inverter to the non-overlapping clock generator. The input inverter 100 has an overlapping clock signal, CK, as an input. CK is connected to the gate of p-channel field effect transistor (PFET) 102 and the gates of n-channel field effect transistors (NFETs) 104, 106, and 108. The source of PFET 102 is connected to the positive supply voltage. The drain of PFET 102 is connected to the output node of inverter 100, signal NC. The sources of NFETs 104, 106, and 108 are connected to the negative supply voltage. The drains of NFETs 104, 106, and 108 are connected to signal NC.

[0012] In one embodiment, PFET 102 has a width-to-length ration of 23.95. NFET 104 has a width-to-length ration of 3.6. NFET 106 has a width-to-length ration of 1.2. NFET 108 has a width-to-length ration of 1.8. Accordingly, the P/N width ratio of inverter 100 as shown in FIG. 1 is approximately 3.6.

[0013] With careful layout of the mask layers used to fabricate inverter 100, NFETs 104, 106, and 108 may be connected in any of the configurations shown in FIGS. 2A-2C by making changes only in the metal mask layers. For example, this may be accomplished by ensuring that the gate of NFET 104 was connected to the gate of NFET 106 in at least one metal layer and no non-metal layers and by ensuring that the gate of NFET 106 was connected to the gate of NFET 108 in at least one metal layer and no non-metal layers and that the negative supply voltage was also capable of being connected to the gates of NFET 106 and NFET 108 with a metal-only connection. Also, although not shown if the figures, the gate of NFET 104 could also be made capable of being disconnected and reconnected in a manner similar to that described for NFETs 106 and 108.

[0014] In an embodiment of the invention, the metal layers referred to above could be chosen to be the top, or near to the top, metal layers. This may allow them to be modified by after-manufacture methods such as by a focused ion beam (FIB) device. These after-manufacture methods would not require the production or relayout of mask layers to change the configuration of inverter 100.

[0015] If the width-to-length ratios for NFETs 104, 106, and 108 described above are used, then FIG. 2A illustrates an inverter 100 configuration that has a P/N width ratio of approximately 3.6. FIG. 2B illustrates an inverter 100 configuration that has a P/N width ratio of approximately 5.0. FIG. 2C illustrates an inverter 100 configuration that has a P/N width ratio of approximately 6.7.

[0016] A component of the delay of inverter 100 is determined by the PIN width ration of inverter 100. Accordingly, the delay of inverter 100 may be adjusted by wiring inverter 100 in the different configurations shown in FIGS. 2A-2C. In general, the larger the P/N ratio, the more delay between CK rising and NC falling. Signal NC falling ultimately causes the output of the non-overlapping clock generator, CK1N, to rise. Therefore, the larger the P/N ratio of inverter 100, the greater the dead time caused by the non-overlapping clock generator.

[0017] Although a specific embodiment of the invention has been described and illustrated, the invention is not to be limited to the specific forms or arrangements of part so described and illustrated. The invention is limited only by the claims. 

What is claimed is:
 1. An apparatus, comprising: a non-overlapping clock generator comprising an inverter that contributes to dead time between a first clock and a second clock; wherein said inverter comprises a switching device of a first type and a plurality of switching devices of a second type; and, wherein said plurality of switching devices of said second type have a plurality of control terminals and said control terminals are connected by at least one metal layer.
 2. The apparatus of claim 1, wherein said switching device of said first type is a PFET.
 3. The apparatus of claim 1, wherein said plurality of switching devices of said second type are NFETs.
 4. The apparatus of claim 1, wherein said non-overlapping clock generator comprises a gater input and said inverter comprises an inverter input and said gater input is connected to said inverter input.
 5. The apparatus of claim 1, wherein said at least one metal layer connects at least one of said control terminals of a first of said plurality of switching devices to another terminal of said first of said plurality of switching devices thereby keeping said first of said plurality of switching devices non-conducting.
 6. The apparatus of claim 5, wherein said at least one metal layer connects at least one of said control terminals of a second of said plurality of switching devices to another terminal of said second of said plurality of switching devices thereby keeping said second of said plurality of switching devices non-conducting.
 7. The apparatus of claim 5, wherein keeping said first of said plurality of switching devices non-conducting increases said dead time between said first clock and said second clock.
 8. The apparatus of claim 6, wherein keeping said second of said plurality of switching devices non-conducting increases said dead time between said first clock and said second clock.
 9. A method, comprising: fabricating a first non-overlapping clock generator on a first wafer wherein said first non-overlapping clock generator generates a first dead time between a first clock and a second clock; holding a second wafer at a process step before fabricating at least one layer of metalization; and, fabricating a second non-overlapping clock generator on said second wafer wherein said second non-overlapping clock generator generates a second dead time between said first clock and said second clock because of changes in said at least one layer of metalization as fabricated on said first wafer and said second wafer.
 10. The method of claim 9, comprising: fabricating a first plurality of switching devices that are part of a first inverter that is part of said first non-overlapping clock generator wherein said first plurality of switching devices have a first plurality of control terminals and said first plurality of control terminals are connected by said at least one layer of metalization.
 11. The method of claim 10, comprising: fabricating a second plurality of switching devices that are part of a second inverter that is part of said second non-overlapping clock generator wherein said second plurality of switching devices have a second plurality of control terminals and at least one of said second plurality of control terminals is connected by said at least one layer of metalization to keep at least one of said second plurality of switching devices non-conducting.
 12. The method of claim 11 wherein keeping at least one of said second plurality of switching devices non-conducting helps make said second dead time greater than said first dead time.
 13. An apparatus fabricated on an integrated circuit, comprising: a non-overlapping clock generator; said non-overlapping clock generator comprising an inverter; said inverter comprising a first switching device and at least a second switching device and a third switching device; said second switching device having a second control terminal; said third switching device having a third control terminal; patterned metal connecting said second control terminal and said third control terminal; and, wherein changing the pattern of said patterned metal can connect said third control terminal to a node that keeps said third switching device non-conducting thereby changing a dead time generated by said non-overlapping clock generator.
 14. The apparatus of claim 13 wherein said first switching device is a PFET.
 15. The apparatus of claim 14 wherein said second switching device and said third switching device are NFETs.
 16. The apparatus of claim 13 wherein said patterned metal is only on one metal layer. 